1. Field of the Invention
The present invention relates to an apparatus for detecting faults on the surface of a resist master disc for producing optical discs and measuring the thickness of the resist coating layer on the master disc.
2. Description of Background Information
The optical recording of information is in practical use as a method in which video and audio information is recorded on a disc shaped recording medium.
In one typical process of producing an optical information recording disc, a resist master disc is prepared by coating a glass disc used as a substrate with a thin layer of a resist material. The master disc is treated in the so called bit by bit system exposure process in which the resist layer is exposed to a finely focused laser beam which is modulated in a blinking manner with the video and audio information to be recorded. Subsequently, the master disc is treated by a development process in which exposed portions of the resist thin layer are removed to form a series of pits which carry the information to be recorded in the form of the length of each pit and its repetition rate.
In optical information recording using optical discs, loss of recorded information generally called as a "drop out," is rather difficult to eliminate. Therefore it is necessary, in the process of producing the master disc, to detect the presence of faults such as dust, defects, or flaws on the surface of the master disc which may cause a drop out. On the other hand, it is also very important that the thickness of the thin resist coating layer on the surface of the glass master disc be maintained constant. Therefore, it is equally necessary to measure the thickness of the resist layer.
Conventionally, for detecting faults on the surface of the master disc and for measuring the thickness of the resist layer, an apparatus for detecting faults on the surface of a resist master disc and an apparatus for measuring the thickness were prepared separately. An example of conventional measuring process is as follows. When the detection of faults by the first apparatus is completed, the master disc is transported, by hand, to the thickness measuring apparatus where the thickness of the resist layer is measured.
Therefore, an increase of the cost was not avoided because of the necessity for preparing two apparatuses, and a long time was required for completing both of the detection of faults and the measurement of the thickness.